Add support for already_declared enums

[chinedufn]

This commit allows enums to use the already_declared attribute.

This is useful when you want to use the same enum across multiple bridge
modules.

---

For example, the following is now possible:

use some_other_bridge_module::SomeEnum;

#[swift_bridge::bridge]
mod ffi {
    #[swift_bridge(already_declared)]
    enum SomeEnum {}

    extern "Rust" {
        fn print(val: SomeEnum);
    }
}